The easiest way to learn, at least for me when I was young, is to hold the index and middle fingers together on each hand and touch the ends of your middle fingers together at a 60 - 80 degree angle to each other, slide them into your mouth about a half inch deep, using the tips of your fingers to curl the tip and about 3/4 inch of your tongue back so that it presses against the rest of your tongue. Tighten your lips around your fingers and exhale. Give it a try, it takes a little finesse, don't get discouraged.
No success, yet. I'll keep trying.
Tell me, is your curled back tongue touching the roof of your mouth? Also, I have seen some people perform this kind of whistle without using a hand. What is the purpose/advantage/whatever, of using your hand?